Surface densification of powder metal bearing caps

ABSTRACT

A high performance main bearing cap has particular surfaces densified for improved fatigue crack resistance. The surfaces densified are the bolted face (F) inside of a perimetral margin and outside of bold head interface areas around the main bolt holes (B), the surfaces of the main bolt holes and the side bolt hole threads (S). Preferred methods of densification are single needle programmable pattern peening of the bolted face peened area, over-burnishing of the bolt holes, and forming the threads rather than cutting them for the side bolt threads.

FIELD OF THE INVENTION

[0001] This invention relates to performance enhancements to powdermetal parts by surface densification, and in particular, to powder metalbearing caps.

BACKGROUND OF THE INVENTION

[0002] The use of powder metallurgy (P/M) to produce steel main bearingcaps for passenger vehicle engines has grown from zero to well over 70million components in service. The material being replaced is cast iron,usually of the type commonly known as “Ductile Cast Iron” (DCI). Thereare many commercial and technical advantages to using the P/M process,including elimination of many costly machining steps, forming of uniqueshapes and geometries during the molding (powder compaction) stage, andmaterial versatility. The large majority of engines used in automotivevehicles fall in a power density ratio (power to engine-size ratio) thatplaces stresses on the main bearing caps that can be accommodated by theP/M steel's inherent material strength. However, there are some specialpurpose high performance engines that are used for special road carsincluding racing that go beyond the normal power density ratios. Inthese special cases, the main bearing cap's performance safety factor isreduced from the preferred minimum of 1.5 to a level approaching 1.0.The 1.0 safety factor means that the component would only just survivethe maximum rated engine performance in the long term.

[0003] In such cases, it is appropriate to enhance the performance(strength under cyclic fatigue conditions) of the main bearing cap toprovide a comfortable safety margin.

[0004] The P/M steel materials used for main bearing caps can bestrengthened by conventional means such as heat-treating of the material(quench-hardening). In this case, the material is inevitably muchharder, and is therefore resistant to the machining operations that arerequired after the component is installed in the engine cylinder block.

[0005] A virtually unique property of metals processed by powdermetallurgy is the capability to vary the density, which is the mass perunit volume of the material. This property naturally develops during theP/M manufacturing process that is well known to those versed in the art.Briefly, this consists of compacting the selected powder mix, under highpressure, in specifically designed tooling, into a shape known as a“pre-form”, which is then thermally treated by a process known as“sintering”, which causes the powder particles to fuse together, therebydeveloping mechanical strength.

[0006] It is also well known to those versed in the art that thephysical and mechanical properties of the P/M metal increase as thedensity of the metal increases.

[0007] Therefore, to increase the strength of a P/M steel main bearingcap without prejudicing the ease of machining (machinability), it isappropriate to raise the density of the compact. This can normally beachieved by raising the powder compaction pressure, but this option islimited by the strength of the compaction tooling. Alternatively, thedesign can be simplified to enable more robust tooling to be designedthat can withstand higher compaction pressure, but this invariably leadsto additional costly machining operations, which may render the productnon-viable commercially.

[0008] A special feature of metals that are at less than full density isthe ability to locally densify the surface by application of mechanicalpressure. This can be achieved in several ways, for example by rolling ahard roller over the surface (burnishing), or by localized hammering(peening). Such local densification processes are known to those versedin the art. These processes, when correctly applied, may also result infavorable “residual compressive surface stresses” that can extend theoperational life of the product under cyclic fatigue conditions.

[0009] This invention teaches a method of incorporating these principlesin a new way to enhance the performance of powder metal mechanicalcomponents, and in particular, a powder metal steel main bearing cap tomeet the demands of modern high performance car engines.

[0010] There are three principal mechanical failure modes associatedwith high performance engine main bearing caps, namely fatigue crackingthrough the bolted face (FIG. 1a), fatigue cracking through the innerbolt hole (FIG. 1b), and side bolt-hole thread failure (FIG. 1c).

[0011] A research program was initiated at the inventor's company todetermine if and how the strength of the main bearing cap could beraised by application of surface densification to each of these criticalareas. This required extensive processing development work, plus manylong term fatigue tests on both test pieces and on actual main bearingcaps that are in current production.

SUMMARY OF THE INVENTION

[0012] In one aspect, the invention provides a high performance bearingcap construction which addresses the main failure modes of a mainbearing cap and a method of making the construction. The inventionsurface densities certain areas of the bearing cap to increase itsstrength and resistance to failure.

[0013] In particular, one aspect of the invention is to surface densifythe bolted surface of the main bearing cap. It is preferred to performthis densification on the bolted surface of the cap inward of each mainbolt hole but not at the areas which are under the heads of the bolts,and not to densify the surface immediately adjacent to the edges of thebolted surface, so a small undensified margin is left at the edges. Thebolt head interface area is not densified to preserve its surface finishfor consistent tightening friction and bolt stretching when the boltsare tightened, and the margin areas are not densified so as not tocreate any sharp edges or burrs at the edges.

[0014] The densification of the bolted surface is preferably performedby needle peening, and specifically by a precision pattern peeningprocess, using a precision programmable single needle peening machine.The single needle peening machine is of the type normally used forstamping numbers into parts, sometimes called a pin stamper. It peenswith a single needle in a matrix pattern, the specific shape of which isprogrammable. Therefore, it can be programmed to surface densifyspecific areas in a powder metal component. In the case of a mainbearing cap, it is programmed to densify an area inside of marginsadjacent to the edges of the bearing cap and on the inner side of thebolt head interface area around each inner bolt hole, which is theinterface between the head of the bolt and the bearing cap. Thedensified area extends inwardly for a length which is sufficient tocover the most likely area of fatigue crack propagation.

[0015] In another aspect of the invention, the bolt holes (which extendthrough the bolted surface) are mechanically expanded to an extent so asto densify the bolt hole surface to a significant depth. Thedensification depth is sufficient to strengthen the bearing cap in thearea of the bolt hole and increase its resistance to developing afatigue fracture that starts at the bolt hole. Preferably, thedensification is performed for the full length of the bolt hole, butshould at least cover the length which is about a third of the way upfrom the bottom end of the bolt hole (the bottom end of the bolt hole isat the surface of the bearing cap which interfaces with the crankcaseand that the bolt goes through).

[0016] In another aspect of the invention, the bearing cap is improvedby forming, rather than cutting, the threads in the side bolt holes ofthe bearing cap, if side bolt holes are provided. Forming the threads,by roll forming for example, densifies the thread surface to a depththat resists stripping or shearing of the threads, or pulling out of theside bolt.

[0017] In an especially useful aspect of the invention, a combination oftwo or more of the identified areas are surface densified. If only onearea is surface densified, then that may only make it more likely thatthe failure will occur in one of the other areas. Preferably, at leastthe bolted face and the bolt holes are surface densified. If all threeareas are surface densified, the three main failure modes are addressed,resulting in a very high performance bearing cap.

[0018] In another aspect, the invention provides a method of surfacedensification of powder metal components within specific areas of asurface by mechanically indenting the area to density the surface of thearea by applying a geometrical pattern of overlapping sphericalindentations within the area. This method results in a specific area ofsurface densification, without adversely affecting other areas of thecomponent.

[0019] These and other objects and advantages of the invention will beapparent from the detailed description and drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0020]FIG. 1a is a side plan view illustrating a fracture R which haspropagated through the bolted face F of a main bearing cap;

[0021]FIG. 1b is similar to FIG. 1a, but illustrating the main boltholes B and the side bolt holes S in hidden lines and showing a fracturewhich has propagated from the inner side of one of the main bolt holesB;

[0022]FIG. 1c is a cross-sectional view showing thread failure in one ofthe side bolt holes S;

[0023]FIG. 2 is a side plan view of a main bearing cap of a type havingan arch or hump in the bolted surface F;

[0024]FIG. 3 is a side plan view of a main bearing cap of a type havinga generally flat bolted surface F (there may be raised or unraised landsaround the bolt holes in either type of bearing cap);

[0025]FIG. 4 is a cross-sectional photo-micrograph of a needle peenedarea of the surface of a powder metal part;

[0026]FIG. 5 is like FIG. 4, but of an unpeened surface;

[0027]FIG. 6a is a view of needle peening overlapping onto a bolt headinterface surface of a bearing cap;

[0028]FIG. 6b is a view of a pattern peened surface illustrating singlepin patterned peening tangent to the bolt head interface surface,without overlapping thereon;

[0029]FIG. 7 is a cross-sectional photo-micrograph showing a burr formedat the edge of a conventionally needle peened surface;

[0030]FIG. 8 is a cross-sectional photo-micrograph of the inside of thebolt hole B after burnishing;

[0031]FIG. 9 is like FIG. 6 but showing the bolt hole B beforeburnishing;

[0032]FIG. 10 is a photographic photocopy showing an orange peel effecton the surface of the bolt hole B resulting from excessive burnishing ofthe bolt hole;

[0033]FIG. 11 is a perspective view of side bolted bearing caps C, eachshown with two side bolts D and two main bolts M;

[0034]FIG. 12 is a cross-sectional photo-micrograph of a formed threadin a powder metal component, showing densification;

[0035]FIG. 13 is a cross-sectional photo-micrograph of cut thread in apowder metal component, showing no densification; and

[0036]FIG. 14 is a graphical comparison of cut thread strength to formedthread strength in the side bolt holes of a P/M main bearing cap.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

[0037] Referring to FIGS. 1a and 1 b, a powder metal bearing cap C has abore arch A in a bridging section G between two legs L of the bearingcap C, with main bolt holes B extending through the legs L from a boltedface F which is opposite from the bore arch A to a joint face J of eachleg L. The joint face J is opposite from the bolted face F, with onejoint face on each side of the arch. As is well known, the cap C isbolted to a crankcase so that the arch A, together with a similar archin the crankcase, defines the bore in which the crankshaft of the engineis journalled.

[0038] The present invention provides improvements to the fatigue crackresistance of powder metal components, in particular in the preferredembodiment to a main bearing cap, by surface densifying certain surfaceswhich are susceptible to failure by fatigue cracking. In the mainbearing cap, there are three such surfaces, as stated above: the boltedsurface F; the main bolt hole surfaces B; and the side bolt threadsurfaces S. Each is discussed below.

Localized Surface Densification of the Bolted Surface

[0039] The bolted surface F of a main bearing cap C may be flat orfeature an arch (compare FIGS. 2 and 3) in the bridging region G whichis between the two legs L. In both cases, the maximum critical stress inservice is a tensile cyclic stress (fatigue stress) in the bridgingregion between the inner main bolt-holes B (note that one, two or moremain bolt holes may be provided in each leg L with one “inner” holeclosest to the arch A and the others further outward). A typical crack Rdeveloped in this mode of failure is shown in FIG. 1a. For thisdevelopment work, a combination of using strain gages on actual capsunder simulated service conditions and finite element analysis (FEA) wasused to determine the location of the maximum tensile cyclical stress. Asimilar procedure was employed upon the fatigue test coupons that wereused to determine basic material property enhancement in fatigue testunits used in the program.

Shot Peening and Needle Peening to Improve Component Fatigue Life

[0040] Shot peening is in general known as a method of improving fatiguestrength. Shot peening involves firing hard shot (small particles)against the surface to be strengthened. This process however is costlyand dusty, requiring extraordinary precautions to prevent localized aircontamination that is potentially harmful and certainly unpleasant tooperators.

[0041] Another drawback to shot peening is that it is indiscriminate andcovers all surfaces exposed to the shot stream. In some cases a shotpeened surface condition is undesirable, since peening roughens thesurface. This can adversely alter the friction coefficient of thesurface, and may also detract from the product's cosmetic appearance. Tobe selective, the areas that would be damaged by the shot peening actionmust be shielded or individually masked and then unmasked. This is avery costly procedure.

[0042] Needle peening is a less well-known alternative and is a processemploying hard steel needles that are caused to hammer (peen) thesurface of the metal. This is a more environmentally friendly processthat is also lower cost.

[0043] The needle peening process is far more localized than shotpeening and can be aimed at the specific area of a component that mustbe strengthened. This process was assessed on the critical regions ofmain bearing caps. The equipment used was a commercially availabledescaling gun. This is usually employed in removing scale from weldedjoints—to permit painting, improve appearance and to reveal the qualityof the weld. The needles are typically 6-8 inches long and about 0.125inches diameter. The tips of the needles commonly used for descalingwere found to be unsuitable for the needle-peening process.Experimentation showed that after a prolonged period (several hours) theoriginal square chamfered tips of the needles assumed a naturalspherical radius that was thereafter quite stable. Therefore animportant part of the invention is to pre-determine this “naturalradius' and to machine the needle tips to this form before starting touse them in service. Failure to do this leads to uneven and sharp needleindentations in the early parts processed. Sharp edged indentationswould not improve fatigue life of the component, and may even reducecomponent service life by providing stress raisers.

[0044] The results of the needle peening trials are shown in thephoto-micrograph of a cross section of a treated surface (FIG. 4) Thisillustrates the local surface densification produced by the radiusedimpacting needles. In contrast, FIG. 5 shows the undensified surface.

[0045] Assessment of the shot peening and the localized air gun needlepeening processes and their respective effect on fatigue life wascarried out in two stages. The first stage used six fatigue life testingmachines. Test coupons (samples) were prepared and were split into threegroups: untreated, shot peened and needle-peened. Then fatigue testingwas carried out on all three sets of test coupons. Both the shot peenedand airgun needle peened coupons yielded an increase in fatigue life ofat least 15% over the untreated coupons. The exact improvements achieveddepended on process parameters selected. These include intensity of shotor needle impact, time of treatment, and diameter of shot or needle tip.These parameters should be optimized for the specific material beingprocessed.

[0046] While the air gun needle peening process was very effective onthe coupons and would be equally effective on many P/M mechanicalcomponents, it was found that there were two drawbacks to the airgunneedle peening when applied to the main bearing cap. Each needle isguided in the airgun barrel by guide holes, but the holes must allowfreedom for the needle to both rotate and also to produce a randompattern of overlapping indentations in order to avoid repeated hammeringin one spot. It is well known that the overlapping of indentations isessential to produce the increase in fatigue life. This occurs naturallyin shot-peening due to the random impact of the shot. With airgunneedle-peening, each needle covers a circle of indentations at least 5times the needle diameter, and this limits the application of theprocess in terms of precision of the perimeter of the treated surface.

[0047] The first example (FIG. 6a) shows how the dispersion of needleindentations N extends onto the bolt head interface surface I that isclamped against the main bolt captive-washer. There is a strict surfacefinish specification for this area I in order to control frictionbetween the bolt head and main bearing cap interfacing surfaces duringapplication of the specified bolt torque parameters. This is critical toachieving a consistent bolt-down load and bolt tension on the enginecylinder-block line at the engine maker's plant. This level ofindentation dispersion can be reduced by individual component shieldingbut that involves extra cost and complexity.

[0048] The second drawback is shown in FIG. 7, where the needle tips hitat the edge of the main bearing cap surface and swage the metal into avery sharp overhanging lip 0. This is unacceptable for safe producthandling, and requires component shielding during peening or additionalprocessing to remove the sharp burr.

The Precision Pattern Peening (3P) Process

[0049] In an effort to overcome the drawbacks described above, analternative process with improved indentation-pattern precision wasneeded. A process was developed that achieved these goals and is calledthe “3P” process which stands for Precision Pattern Peening. The processinvolves use of a programmable-pattern single needle indenting machine.The machine used for development is employed, in its standard form, forinscribing identification characters on metal surfaces. Such a machineis commercially available, for example, from Telesis Technologies, Inc.of Circleville, Ohio. By increasing the power of the machine and byfitting a special holder and a precisely machined tungsten carbideindenter, it proved possible to produce precisely controlled overlappingindentation patterns within a precise perimeter. This approach solvedall the outstanding problems associated with both shot peening andairgun needle peening.

[0050]FIG. 6b shows the distinctive geometrical indentation pattern Dfrom the 3P process. This pattern is very distinctive, and in starkcontrast to the random pattern produced by previously known peeningprocesses. This contrast is shown by comparing FIGS. 6a and 6 b. Asshown in FIG. 6b, the 3P process precisely limits the boundary of thepeened area to avoid the bolt head region I and also leaves a smallunpeened margin T adjacent to each side edge of the bearing cap C, whichavoids generation of the sharp edge burr associated with theconventional peening processes. The length P of each densified area D onthe bolted face F of the pattern peened cap C is shown in FIGS. 1b and 6b, illustrating that the densified area D covers the two areas of thebolted face F from which fatigue cracks involving the bolted face aremost likely to propagate. The pattern peener peened this area to a depthof approximately 0.0635 mm (measured from the surface of an unpeenedarea to the surface of a peened area). It is noted that two singleneedle pattern peeners could be used to densify a single bearing cap,one peener working on the right area D and the other working on the leftarea D, to reduce cycle time.

[0051] The effectiveness of the 3P process to extend fatigue life wasassessed by going directly to fatigue testing of main bearing caps onfatigue testing machines. By adjusting the pattern laid down by the 3Pprocess to provide controlled overlapping and density of indentations(which may be varied, depending on the material being peened), thefatigue life of the main bearing cap was increased by at least 15%.Remarkably, the time cycle to produce an acceptable pattern over thetargeted area was actually faster than both of the other traditionalprocesses.

[0052] An additional benefit of the 3P process is the quality controlaspect in large scale production. The automated setup is far moreconsistent than either shot or air-gun needle peening. The onlyuncontrolled variable is the wear rate of the single carbide indenter.By measuring the change in profile of the single indenter using astandard profile comparator, the indenter life can be predicted and theindenter changed under preventive maintenance procedures.

[0053] It is important to note that the powder metal material used forthis research has a ductility of 3% tensile elongation. This inventionmay be less effective on brittle materials since they will be prone tomicro-cracking.

Local Densification of the Bolt Hole by Hole Surface Densification

[0054] Careful examination of deliberately failed main bearing caps(from over-stressing) determined that a fatigue crack initiated on theinside of the inner bolt hole B at about a third of the height from thejoint face J at the thinnest wall section. A typical crack X developedin this mode of failure is shown in FIG. 1b.

[0055] A well-known method of improving the surface finish of a hole ina traditional metal component is to “burnish” the inner surface with aburnishing tool. The tool consists of hard pins that act as rollers thatsmooth the surface, removing rough areas and improving hole roundness.Typical burnishing of P/M holes results in expansion of the diameter ofthe hole by 0.025 to 0.050 mm. In this invention, burnishing tools areapplied to expand the hole to a much greater extent, beyond theconventional smoothing action, to effect substantial surfacecompression, which results in surface densification. Experimentation onholes in P/M steel has shown that the limiting degree of surfacedensification depends upon the material ductility and starting density,but the limit corresponds to the point where surface integrity breaksdown and circular cracks, sometimes called orange-peeling, begin toform. Thus, in practicing the invention, the bolt hole surfaces arecompressed significantly more than would normally be done in burnishingto smooth the surface, remove rough areas and improve hole roundness;enough to compress the hole surface so as to densify it, but less thanthe amount that results in cracking of the surface.

[0056] To evaluate the efficiency of this technique, a powder metalbearing cap was selected, and the most highly stressed main bolt-holeswere progressively burnished to increasing degrees.

[0057] It was found that applying the “normal” degree of surfaceburnishing, as recommended in technical publications, did not causesurface densification. However, by “over-burnishing” to well beyond therecommended level, significant densification occurred. This is shown inFIG. 8, which is a cross section through a bolt hole B over-burnished by0.15 mm on diameter (from a diameter of approximately 10.85 mm to adiameter of 11.00 mm). FIG. 9 shows the density without over-burnishing.While this amount of hole surface densification achieves near maximumadvantage, as little diameter expansion as 0.10 mm would achievesignificant advantage from the invention. Therefore, the inventioncontemplates hole wall expansion of from 0.10 mm to the limit at whichsurface cracking starts to form to densify the hole surfaces.

[0058]FIG. 10 shows an excessively over-burnished bolt-hole B that hassuffered from excessive deformation, leading to orange peeling of thesurface. The optimum degree of over-burnishing is dependent on thepowder metal material ductility and starting density. Simulated enginetesting of plain and over-burnished bearing caps resulted in an increasein safety factor from 1.5 to 1.75. This corresponds to a 17% improvementin fatigue resistance.

Combining the 3P and Hole Densification Processes

[0059] In one instance, the 3P process eliminated fatigue failure at thebolted surface of a particular over-stressed main bearing cap, but thefailure site then moved to the inner bolt hole. When the combination of3P and bolt hole densification by over-burnishing was used, the resultwas an even stronger main bearing cap.

[0060] Therefore, it is essential to determine the “weakest link” areaof the bearing cap under severe cyclic stressing, and to then apply theappropriate treatments, alone or in combination.

Local Densification of Side-Bolt Threads by Enhanced Thread-Forming

[0061] In high performance design of main bearing caps, side-bolts D aregenerally used to stiffen the cylinder block (FIGS. 1c and 8). Thisstiffening helps strengthen the cap and also reduces undesirable noiseknown as NVH (noise, vibration, and harshness). These side bolts D drawthe side-walls of the engine block inward to form a solid boltedassembly. The stresses on these bolts can be high enough to strip thethreads of the P/M steel (FIG. 1c). Thus, the failure mode of thesethreads is not fatigue, but stripping during the assembly process, andthat is the problem addressed by this aspect of the invention.

[0062] The conventional way of producing threads in a bolt hole is bythread cutting using a tool called a “tap”. An alternative less commonmethod of producing bolt hole threads is by thread forming. This iswhere the material is deformed into a thread form instead of cutting. Inconventional (non P/M) steel, the process has limitations due to veryhigh stresses that are associated with severe deformation of a solidmaterial. It is difficult to achieve a full thread form without risk oftool breakage.

[0063] It was discovered that if instead of cutting the threads in theP/M material with a tap, the threads were formed by deformation, asignificant degree of thread densification was possible. The naturalmicro-porosity from the P/M process collapses on itself (FIG. 9) toproduce a more dense thread surface. To achieve this condition, it wasnecessary to go well beyond the normal degree of deformation used insolid materials. In so doing it was found that a fully formed thread waspossible without the risk of tool breakage.

[0064] Testing showed that the densification achieved from enhancedthread forming significantly increased the resistance of the P/Mmaterial to thread failure.

[0065] Simulated product testing of main bearing caps with roll formedthreads showed a dramatic improvement in thread strength. The chart inFIG. 10 shows the improvement from 71 N-m to 102 N-m, which is a 44%gain in thread strength.

What we claim is:
 1. In a powder metal bearing cap which has a bore archin a bridging section between two legs of said bearing cap, with boltholes extending through the legs from a bolted face of said bearing capwhich is opposite from said bore arch to a joint face of each leg, whichis opposite from said bolted face, with one joint face on each side ofsaid arch, the improvement wherein at least a portion of said boltedface is surface densified.
 2. The improvement of claim 1, wherein saidsurface is densified by peening.
 3. The improvement of claim 2, whereinsaid peening is needle peening.
 4. The improvement of claim 1, whereinsaid bolted face is surface densified within a precise area of saidbolted face.
 5. The improvement of claim 4, wherein said area is surfacedensified by pattern peening.
 6. The improvement of claim 1, whereinsaid bolted face is not surface densified at bolt head interface areasaround the bolt holes.
 7. The improvement of claim 1, wherein saidbolted face is not surface densified at a margin adjacent to edges ofsaid bolted face.
 8. The improvement of claim 1, wherein said bolt holesare surface densified.
 9. The improvement of claim 8, wherein said boltholes are surface densified by compressing the hole surfaces to expandthe diameter of the holes.
 10. The improvement of claim 1, wherein saidbearing cap includes side bolt holes in end surfaces of the bearing cap,and wherein said side bolt holes have formed threads.
 11. Theimprovement of claim 1, wherein said bolt holes are surface densifiedand said bearing cap includes side bolt holes in end surfaces of thebearing cap, and wherein said side bolt holes have formed threads. 12.The improvement of claim 11, wherein said bolted face is surfacedensified by patterned peening, said bolt holes are surface densified bycompression of the hole surfaces to expand the diameter of the holes.13. In a powder metal bearing cap which has a bore arch in a bridgingsection between two legs of said bearing cap, with bolt holes extendingthrough the legs from a bolted face of said bearing cap which isopposite from said bore arch to a joint face of each leg, said jointface being opposite from said bolted face with one joint face on eachside of said arch, the improvement wherein said bolt holes are surfacedensified.
 14. The improvement of claim 13, wherein said bolt holes aresurface densified by compressing the surfaces of the holes so as toexpand the diameter of each hole by at least 0.10 mm.
 15. Theimprovement of claim 13, wherein said bolt holes are surface densifiedat least adjacent to the joint face.
 16. The improvement of claim 13,wherein said bolt holes are surface densified over substantially theirwhole length.
 17. The improvement of claim 13, wherein said bearing capincludes side bolt holes in end surfaces of the bearing cap, and whereinsaid side bolt holes have formed threads.
 18. In a powder metal bearingcap which has a bore arch in a bridging section between two legs of saidbearing cap, with bolt holes extending through the legs from a boltedface of said bearing cap which is opposite from said bore arch to ajoint face of each leg, said joint face being opposite from said boltedface with one joint face on each side of said arch, the improvementwherein said bearing cap includes side bolt holes in end surfaces of thebearing cap, and wherein said side bolt holes have formed threads.
 19. Amethod of increasing the resistance of a powder metal component tofatigue cracking comprising the steps of identifying a surface area ofthe component which is susceptible to fatigue cracking and mechanicallyindenting the area to densify the surface of the area by applying ageometrical pattern of overlapping spherical indentations within thearea.
 20. The method of claim 19, wherein the component is a bearingcap.
 21. The method of claim 20, wherein the surface area is on a boltedface of the bearing cap.
 22. The method of claim 21, wherein bolt holesof the bearing cap are surface densified by compressing the surfaces ofthe holes to expand the diameter of each hole.